He hasn't set a retirement date yet, but Roger Federer admits he knows it's coming up very soon.

The World No.2, who could yet ascend to the top of the rankings by the end of the year should Rafael Nadal's latest injury keep him out long term, has revealed a set list of things that have to go right in order for him to stay in the professional game.

However, Federer did reveal that if even one of those four things is no longer working out then he may have to re-evaluate his future in the sport.

"I know (retirement) is sooner or later," the Swiss star confessed.

"I know I'm not 22 anymore, but I don't have a date.

"If the body allows me to play, my family allows me to play, if the success is still there, if I'm happy to travel, I will continue.

"But the four things have to work. If one of the four doesn't work it maybe is the time to stop. But, for now, it's okay."

The 19-time Grand Slam champion also suggested that he may have to play less to prolong his career further, in line with his decision not to play in this year's clay court events.

"For me, I have come to realise that sometimes less is more," he added.